On Apr 15, 2017, at 4:44 PM, Andrew Lunn <and...@lunn.ch> wrote:
> Yet i'm debugging an application which expects a timeout even when
> there are 0 packets.
Well, you've already found a bug - it expects a timeout where there are no
packets.
To quote the pcap man page (this is the latest version, which calls it the
"packet buffer timeout" rather than the "read timeout"):
packet buffer timeout
If, when capturing, packets are delivered as soon as they
arrive, the application capturing the packets will be woken up
for each packet as it arrives, and might have to make one or
more calls to the operating system to fetch each packet.
If, instead, packets are not delivered as soon as they arrive,
but are delivered after a short delay (called a "packet buffer
timeout"), more than one packet can be accumulated before the
packets are delivered, so that a single wakeup would be done for
multiple packets, and each set of calls made to the operating
system would supply multiple packets, rather than a single
packet. This reduces the per-packet CPU overhead if packets are
arriving at a high rate, increasing the number of packets per
second that can be captured.
The packet buffer timeout is required so that an application
won't wait for the operating system's capture buffer to fill up
before packets are delivered; if packets are arriving slowly,
that wait could take an arbitrarily long period of time.
Not all platforms support a packet buffer timeout; on platforms
that don't, the packet buffer timeout is ignored. A zero value
for the timeout, on platforms that support a packet buffer time-
out, will cause a read to wait forever to allow enough packets
to arrive, with no timeout.
NOTE: the packet buffer timeout cannot be used to cause calls
that read packets to return within a limited period of time,
because, on some platforms, the packet buffer timeout isn't sup-
ported, and, on other platforms, the timer doesn't start until
at least one packet arrives. This means that the packet buffer
timeout should NOT be used, for example, in an interactive
application to allow the packet capture loop to ``poll'' for
user input periodically, as there's no guarantee that a call
reading packets will return after the timeout expires even if no
packets have arrived.
The packet buffer timeout is set with pcap_set_timeout().
Note especially the next-to-last paragraph - which was put in there long before
TPACKET_V3, to cover, for example, Solaris.
The purpose of the timeout is to make sure packets don't stay stuck in the
kernel buffer for an indefinite period of time (if they're not arriving at a
sufficient rate to fill up the buffer in a reasonably-short period of time);
it's *not* to make sure the application doesn't remain blocked for an
indefinite period of time.
